1. Field of the Invention
The present invention relates to optical head devices and optical information devices (optical information devices) for recording, reproducing, and erasing information stored on an optical information medium such as an optical disk, recording and reproducing methods for optical information devices, systems in which the same are adopted, and objective lenses, diffraction elements, and composite objective lenses, in which an objective lens and a diffraction element are combined, used in optical head devices.
2. Description of the Related Art
Optical memory technologies that employ optical disks having a pit-shaped pattern as high-density, large-capacity storage media have found increasing application, and have been put to practical use for digital audio disks, video disks, text file disks, and data files. The functions that allow such applications to be executed successfully with high reliability with respect to recording and reproducing information to and from an optical disk using a finely focused light beam can be broadly divided into a light-focusing function for forming fine spots at the diffraction limit, the focus controls (focus servo) and the tracking controls of the optical system, and the detection of pit signals (information signals).
Recent advances in optical system design technologies and the development of shorter wavelength semiconductor lasers as light sources have led to progress in the development of optical disks having storage capacities with a higher density than was the case conventionally. Increasing the numerical aperture (NA) on the optical disk side of the light-focusing optical system for finely focusing a light beam onto an optical disk has been investigated as one way to achieve higher densities. The problem with this is that it increases the amount of aberration that is generated due to tilting of the optical axis. When the NA is increased there is greater aberration generated with respect to the tilt. The thickness of the substrate of the optical disk (the substrate thickness) can be reduced to prevent this.
Compact disks (CDs), which may be considered the first generation of optical disks, use infrared light (wavelength λ3 of 780 nm to 820 nm) and an objective lens with an NA of 0.45, and the substrate thickness of the disks is about 1.2 mm. DVDs, the second generation of optical disks, use red light (wavelength λ2 of 630 nm to 680 nm; standard wavelength 650 nm) and an objective lens with an NA of 0.6, and the substrate thickness of the disks is 0.6 mm. A third generation of optical disks uses blue light (wavelength λ1 of 390 nm to 415 nm; standard wavelength 405 nm) and an objective lens with an NA of 0.85, and the substrate thickness of the disks is 0.1 mm.
It should be noted that in this specification the substrate thickness designates the thickness from the surface where the light beam is incident on the optical disk (or information medium) to the information recording surface.
Thus, the thickness of the substrate of high-density optical disks has been reduced. Taking into account the price and the space occupied by the device, it is preferable that optical information devices can both record to and reproduce from a plurality of different types of optical disks with different substrate thicknesses and recording densities. This requires an optical head device that is provided with a light-focusing optical system that is capable of focusing a light beam down to the diffraction limit onto optical disks having different substrate thicknesses.
To record to and reproduce from disks with thicker substrates, it is necessary to focus the light beam onto a recording surface that is deeper than the disk surface, and thus the focal point must be made longer.
JP H7-98431A (Patent Document 1) (FIG. 1) discloses a configuration aimed at achieving an optical head device that records to and reproduces from a plurality of different types of optical disks with different substrate thicknesses. This serves as a first conventional example, and is described using FIG. 11A and FIG. 11B. Reference numeral 40 denotes an objective lens and 41 denotes a hologram. The hologram 41 is constituted by a substrate that is transparent to an incident light beam 44, and its grating pattern is made of concentric circles.
The objective lens 40 has a numerical aperture NA of at least 0.6, and as shown in FIG. 11A, it has been configured so that a zero-order diffraction beam 42 that has passed through the hologram 41 without being diffracted can be turned into a focus spot at the diffraction limit on an optical disk 10, which has a substrate thickness (t2) of 0.6 mm, for example. FIG. 11B shows how a focus spot can be formed at the diffraction limit on an optical disk 11, which has a thick substrate (thickness t1=1.2 mm). Positive first-order diffraction light 43 that has been diffracted by the hologram 41 is focused onto the optical disk 11 by the objective lens 40. Here, the positive first-order diffraction light 43 is subjected to aberration correction so that it passes through the substrate with the thickness t1 and is focused to the diffraction limit.
Combining the hologram 41, which diffracts the incident light, and the objective lens 40 in this manner allows a double focus lens that utilizes the diffraction light beams 42 and 43, which have different orders, to form focus spots that are focused down to the diffraction limit on the optical disks 10 and 11, respectively, which have different substrate thicknesses (t1 and t2), to be achieved. Apart from the above description, Patent Document 1 also discloses the reduction of focal position fluctuation with respect to wavelength fluctuation when recording to and reproducing from the optical disk 10, which has the thickness t2, by giving the hologram 41 a convex lens effect so as to use the zero-order diffraction light for the optical disk 11 whose thickness is t1, and use the positive first-order diffraction light for the optical disk 10 whose thickness is t2.
In addition to the above, there also has been disclosed a configuration whose object is to allow different types of optical disks to be interchangeably reproduced using a plurality of light beams with different wavelengths. A configuration in which a wavelength selection phase plate is combined with an objective lens is disclosed in JP H10-334504A (pages 7 to 9, FIGS. 1 to 4) (Patent Document 2) and in ISOM2001 Technical Digest Session We-C-05 (Proceedings, page 30) (non-Patent Document 1), and serves as a second conventional example. The configuration disclosed in non-Patent Document 1 is described using FIG. 12 and FIG. 13. FIG. 12 schematically shows the configuration of a conventional optical head device. Parallel light that has been emitted by a blue optical system 51, which has a blue light source with a wavelength λ1 of 405 nm, passes through a beam splitter 161 and a wavelength selection phase plate 205 and is focused on an information recording surface of an optical disk 9 (third generation optical disk) having a substrate thickness of 0.1 mm by an objective lens 50. The light that is reflected by the optical disk 9 returns over the opposite path and is detected by a detector of the blue optical system 51.
Divergent light emitted by a red optical system 52, which has a red light source with a wavelength λ2 of 650 nm, is reflected by the beam splitter 161, passes through the wavelength selection phase plate 205, and is focused on the information recording surface of the optical disk 10 (second generation optical disk: DVD), which has a substrate thickness of 0.6 mm, by the objective lens 50. The light that is reflected by the optical disk 10 returns over the opposite path and is detected by a detector of the red optical system 52.
The objective lens 50 is configured so that incident parallel light is focused, passing through the 0.1 mm thick substrate, and when recording to and reproducing a DVD, it generates spherical aberration due to the difference in the substrate thickness. To correct this spherical aberration, the light beam that is emitted by the red optical system 52 is turned into divergent light, and the wavelength selection phase plate 205 is used. New spherical aberration is generated when the divergent light is incident on the objective lens 50, and thus the spherical aberration that is generated due to the difference in substrate thickness is canceled out by this new spherical aberration, and also, the wave front is corrected by the wavelength selection phase plate 205.
FIG. 13A and FIG. 13B show a plan view and a side view of the wavelength selection phase plate 205, respectively. The wavelength selection phase plate 205 is made of a phase step 205a having a height h and a height 3h, if the refractive index at the wavelength λ1 is n1 and h=λ1/(n1−1). For light of the wavelength λ1, the light path difference that is caused by the height h is the used wavelength λ1, corresponding to a phase difference of 2π, which is the same as a phase difference of 0. Thus, there is no effect on the phase distribution or the recording to and reproducing the optical disk 9. On the other hand, for light of the wavelength λ2, when n2 is the refractive index of a phase plate 206 at the wavelength λ2, then h/λ2×(n2−1)≈0.6λ, that is, a light path difference that is not an integer multiple of the wavelength is generated. The phase difference resulting from this light path difference is utilized to perform aberration correction as discussed above.
As a third conventional example, a configuration in which a plurality of objective lenses are employed and are mechanically switched between also has been disclosed (for example, JP H11-296890A (pages 4 to 6, FIG. 1) (Patent Document 3).
As a fourth conventional example, a configuration in which a mirror provided with a reflective surface having a different radius of curvature also serves as a reflecting mirror that bends the optical axis has been disclosed (for example, JP H11-339307A (pages 4 and 5, FIG. 1) (Patent Document 4)).
As a fifth conventional example, a configuration has been disclosed in which, like in the first conventional example, a refraction-type objective lens and a hologram are combined, and chromatic aberration occurring in the same order of diffraction light of light of different wavelengths is employed to correct differences in the substrate thickness (for example, JP 2000-81566A (pages 4 to 6, FIGS. 1 and 2) (Patent Document 5)).
The lecture proceedings 27p-YD-5 of the 63rd Meeting of the Japan Society of Applied Physics and Related Societies (September 2002, Niigata University) (non-Patent Document 2) will be described as the fifth conventional example. The BD and DVD, which employ a blue light source and a red light source, respectively, are substantially identical to those of the second conventional example described using FIG. 12. These are described using FIG. 14. Parallel light emitted from a blue optical system 51 having a blue light source for light with a wavelength λ1 of 405 nm passes through two beam splitters 161 and a wavelength selection hologram 207 and is focused on the information recording surface of an optical disk 9 (third generation optical disk) having a substrate thickness of 0.1 mm by an objective lens 50. Light that is reflected by the optical disk 9 returns over an opposite path and is detected by a detector of a blue optical system 51.
Divergent light emitted by a red optical system 52, which has a red light source with a wavelength λ2 of 650 nm, is reflected by the beam splitters 161 and is focused on the information recording surface of the optical disk 10 (second generation optical disk: DVD), which has a substrate thickness of 0.6 mm, by the objective lens 50. The light that is reflected by the optical disk 10 returns over an opposite path and is detected by a detector of the red optical system 52.
The objective lens 50 is configured so that incident parallel light passes through the 0.1 mm thick substrate and is focused, and when recording to and reproducing a DVD, it generates spherical aberration due to the difference in substrate thickness. To correct this spherical aberration, the light beam that is emitted by the red optical system 52 is turned into divergent light. New spherical aberration is generated when the divergent light is incident on the objective lens 50, and thus the spherical aberration that is generated due to the difference in substrate thickness and is cancelled out by this new spherical aberration.
Moreover, in the fifth conventional example, parallel light that is emitted from an infrared optical system 53, which emits light with a wavelength λ3 of 785 nm, is converted into diffused light by the wavelength selection hologram 207, which has a concave lens effect only on the wavelength λ3, correcting the spherical aberration that is caused by the difference in substrate thickness between the optical disk 11 and the optical disk 9.
The first conventional example discussed above proposes at least the following three inventive concepts: first, the use of diffraction by a hologram to achieve compatibility with optical disks having different substrate thicknesses; second, changing the design of the inner and outer circumference so as to form focus spots with different numerical apertures; and third, the use of diffraction by the hologram to change the focal position of the focus spot for optical disks with different substrate thicknesses. These inventive concepts do not limit the wavelengths of the light beams emitted by the light sources.
Here DVDs, which are second-generation optical disks, include two-layered disks having two recording surfaces. The recording surface on the side near the objective lens (the first recording surface) must allow light to pass through to the surface that is away from the objective lens, and thus its reflectance is set to about 30%. However, this reflectance is assured only for red light, and is not assured for other wavelengths. Consequently, to reliably reproduce a DVD it is necessary to use red light (wavelength λ2=630 to 680 nm). Also, to sufficiently reduce the focus spot radius when recording to and reproducing from third generation disks it is necessary to use blue light (wavelength λ1=390 to 415 nm). The first conventional example does not specifically disclose a configuration that further increases the light usage efficiency when both red light and blue light are used in order to be compatible with different types of optical disks.
Also, although the first conventional example discloses a configuration in which the hologram is provided with a convex lens shape and positive first-order diffraction light is used to reduce movement of the focal position due to changes in the wavelength with respect to a single type of optical disk, it does not disclose a configuration in which movement of the focal position due to changes in the wavelength is reduced with respect to two or more types of optical disks.
In the second conventional example, a wavelength selection phase plate is used as a compatibility element. When recording to and reproducing from a disk with a thick substrate, the distance from the recording surface to the objective lens is increased by the substrate thickness, and thus it is necessary to extend the focal length. The focal length can be extended by providing the compatibility element with a lens power, but the wavelength selection phase plate does not have a lens power. Also, when the red light is turned into diffused light as in the second conventional example in order to achieve all of this lens power, a large aberration occurs when the objective lens is moved for tracking, for example, and this causes the problem of a decline in recording and reproducing properties.
In the third conventional example, the objective lens is switched and thus a plurality of objective lenses are required, which increases the number of components as well as makes it difficult to provide a compact optical head device. Also, the fact that a switching mechanism is required itself is problematic because it makes it difficult to provide a compact optical head device.
In the fourth conventional example, the objective lens is driven independent of the mirror (see FIGS. 4 to 6 of Patent Document 4). However, since the light beam is converted from parallel light by a mirror having a radius of curvature, as discussed above, moving the objective lens for the purpose of tracking, for example, changes the position of the objective lens relative to the wave front of the incident light, which causes aberration and worsens the light-focusing properties.
Also, the reflective surface of the mirror is constituted by a surface having a radius of curvature, that is, a spherical surface, but with a spherical surface, there is the problem that differences in substrate thickness and differences in wavelength cannot be corrected sufficiently, and high-order aberration of a fifth order or higher cannot be reduced sufficiently.
In the fifth conventional example, a wavelength selection hologram that has a concave lens effect only with respect to a 785 nm wavelength is used in order to reproduce optical disks with a 1.2 mm substrate thickness (CDs); however, the configuration of the wavelength selection hologram is not disclosed in detail. Also, as regards red light and blue light, it is conceivable to constitute a hologram using a phase step that results in a phase of an integer multiple (three or more times) of a specific wavelength, such as red light (660 nm) and blue light (405 nm), but by designing the wavelength selection hologram 207 considering only the reproduction of the optical disk 11 with infrared light, even slight fluctuations in the wavelength, such as the red light becoming 661 nm due to temperature changes, will cause aberration reaching several dozen mλ rms. Thus there is the problem that it may not be possible to record to or reproduce from the optical disk 9 and the optical disk 10.
Accordingly, the present invention was arrived at in light of the foregoing problems, and it is an object thereof to interchangeably reproduce and record to an optical disk (CD) with a 1.2 mm substrate thickness and a corresponding wavelength λ3 (typically approximately 790 nm), an optical disk (DVD) with a 0.6 mm substrate thickness and a corresponding wavelength λ2 (typically approximately 650 nm), and an optical disk (blue light disk) with a 0.1 mm substrate thickness and a corresponding wavelength λ1 (typically approximately 405 nm) with excellent light usage efficiency, and using a single objective lens.